Method for Blast Vibration Monitoring - Guidelines

IS 14881: Scope - Key Points, Formulas & Tables

Scope Summary:

IS 14881 provides guidelines for blast vibration monitoring, focusing on safe limits for air over-pressure and vibration to prevent structural damage during blasting operations.

Key Specifications:

• Rounding Off Values:
  Follow IS 2:1960 for rounding test results, keeping the same significant figures as specified.

• Blast Attenuation Curves (Clause 6.2):
  Use Curve P for pre-splitting, cratering, and new bench levels.
  Site-specific attenuation relations should be obtained from tests for accurate safe charge weight per delay.

• Safe Air Over-Pressure Limits (Clause 16.3):
  Limits relate to wall strain equivalent to 19 mm/s peak particle velocity (PPV).
  Broken glass usually occurs at 136-140 dB air over-pressure.

Table 1: Air Over-Pressure Control Limits (Clause 16.3)

• Note: C-weighted scales require stricter limits due to less sensitivity at low frequencies.

Formula (General attenuation relation concept):

[ PPV = \frac{K \times (W)^{\alpha}}{(D)^{\beta}} ]

Where:

• (PPV) = Peak Particle Velocity (mm/s)
• (W) = Charge weight per delay (kg)
• (D) = Distance from blast (m)
• (K, \alpha, \beta) = Site-specific constants (Curve P values used if unknown)

flowchart LR
    A[Blast Operation] --> B[Charge Weight per Delay (W)]
    B --> C[Distance from Blast (D)]
    C --> D[Calculate PPV using attenuation relation]
    D --> E{PPV < Safe Limit?}
    E -->|Yes| F[Safe Blasting]
    E -->|No| G

IS 14881: Range of Blast Effects — Key Points & Formulas

1. Attenuation Relation (Clause 15.2 & 6.2)

• Blast vibration attenuation depends on site geology, blast geometry, and timing.
• Larger burden → attenuation curve parallel to Fig. 3 but with higher velocity intercept.
• Use Curve P (Fig. 3) for:
  • Pre-splitting
  • Cratering
  • New bench levels
  • Conservative shot design without monitoring

2. Scaled Distance Formula

[ R_s = \frac{R}{W^{1/2}} ] Where:

• ( R_s ) = Scaled distance (m/kg(^{1/2}))
• ( R ) = Distance from blast (m)
• ( W ) = Explosive charge weight (kg)

3. Particle Velocity & Frequency (Clause 6.3)

• Particle velocity decreases with distance.
• Dominant frequency lowers with distance; lower frequencies cause greater structural response.
• At large distances, surface waves dominate over body waves.

4. Safety Criteria (Fig. 3 Summary)

• Attenuation curves show scatter due to geology & blast design.
• Confined shots (e.g., pre-splitting) produce higher peak velocities.

Visual Summary (Fig. 3 Concept)

graph LR
A[Explosive Charge Weight (W)] --> B[Scaled Distance Rs = R / W^0.5]
B --> C[Particle Velocity]
C --> D[Structural Response]
B --> E[Attenuation Curve (Curve P)]
E --> F[Safe Shot Design]

Use site-specific blast tests to refine attenuation relations for accurate safety and design.

IS 14881: Character of Blast Excitation - Key Points

1. Wave Types from Blasting (Clause 4.3)

• Body Waves (P/S waves):
  • P = Primary (compressional) waves
  • S = Secondary (shear) waves
  • Travel through earth materials in spherical propagation near the blast source.
• Surface Waves (R waves):
  • Rayleigh waves travel along ground surfaces/interfaces
  • Become dominant at larger distances due to energy conversion at boundaries.

2. Ground Motion Components (Clause 4)

• Three mutually perpendicular components:
  • L (Longitudinal): Along blast-to-sensor line
  • T (Transverse): Horizontal, perpendicular to L
  • V (Vertical): Vertical direction
• For structural response, axes are often H1, H2 (horizontal principal axes), and V.

3. Dominant Frequency Estimation (Clause 5)

• Methods:
  • (a) Visual inspection of time history
  • (b) Response spectra analysis
  • (c) Fourier frequency spectra

4. Structural Response and Excitation Velocity (Clause 3)

• Excitation particle velocity correlates with observed cracking.
• Full waveform time histories are essential for accurate response prediction.
• Strain at critical structural locations is the best blast response descriptor.

Summary Table: Blast Wave Characteristics

flowchart LR
    Blast --> BodyWaves(P/S)
    BodyWaves --> P(Compressional)
    BodyWaves --> S(Shear)
    Blast --> SurfaceWaves(Rayleigh)
    BodyWaves --> Boundary[Boundary (rock/soil/surface)]
    Boundary --> SurfaceWaves

Note: IS 14881 emphasizes recording full waveforms and

IS 14881 Key Points on Ground Motion:

1. Ground Motion Components (Clause 4)

• Three mutually perpendicular components:
  • L (Longitudinal): Along blast-to-transducer line (horizontal)
  • T (Transverse): Perpendicular horizontal direction
  • V (Vertical)
• Peak ground motion = maximum vector sum of L, T, V components at the same instant.
• True max vector sum is 5-10% greater than max single component peak.
• Pseudo max vector sum (adding maxima of each component irrespective of time) can overestimate by up to 40%.

2. Preferred Descriptor (Clause 9.1)

• Particle velocity is preferred for vibration control and correlation with blast-induced cracking.
• Displacement can be found by integrating velocity.
• Acceleration should be measured directly (not by differentiating velocity) to avoid phase errors.

3. Human Response (Clause 2.3)

• Ground motion and air-borne disturbances cause audible noise and wall rattling.
• Human perception is sensitive to these vibrations and noise, influencing damage reports.

4. Vector Sum Formula

[ V_{max} = \max_t \sqrt{L(t)^2 + T(t)^2 + V(t)^2} ]

• Where (L(t), T(t), V(t)) are instantaneous velocities in each component.

Summary Table: Ground Motion Parameters

graph LR
    Blast -->|Excites| Ground_Motion[L, T, V Components]
    Ground_Motion -->|Particle Velocity| Structural_Response
    Ground_Motion -->|Noise & Vibration| Human_Response
    Structural_Response -->|Cracking| Damage_Assessment

**Use particle velocity time histories for accurate vibration control

Blast-Induced Air Over-Pressure (IS 14881: Clause 16.3, 7.2, 7.3)

Key Formulas

• Decibel Level of Air Over-Pressure:

[ dB = 20 \log_{10} \left(\frac{P}{P_0}\right) ]

• (P) = measured peak pressure (Pa)
• (P_0 = 20 \times 10^{-6}) Pa (reference pressure)

Air Over-Pressure Control Limits (Table 16.3)

¹ C-weighed has almost flat low-frequency response but requires most restrictive limits due to insensitivity to low frequencies.

Important Notes

• Structures respond mainly to low-frequency air over-pressures, which are often inaudible.
• Airblast overpressure scales with cube-root distance scaling.
• Broken glass typically occurs at 136-140 dB (linear transducer).
• Using a wall-strain equivalent to 25 mm/s particle velocity increases allowable overpressure by 3 dB.
• A-weighted scales are not suitable for blast air overpressure monitoring.

Conceptual Diagram: Air Over-Pressure Wave and Structural Response

graph LR
    Blast_Explosion --> Air_Overpressure_Wave
    Air_Overpressure_Wave -->|High Frequency (Audible)| Noise
    Air_Overpressure_Wave -->|Low Frequency (Inaudible)| Structural_Excitation
    Structural_Excitation --> Secondary_Noise
    Structural_Excitation --> Possible_Window_Damage

This summary provides essential formulas, limits, and understanding of blast-induced air overpressure per IS 14881.

IS 14881: Scaling and Attenuation Relations Key Points

1. Attenuation Relations (Clause 6.2 & Fig. 3)

• Curve P: Used for pre-splitting, cratering, and new bench levels.
• Reflects typical scatter due to geology and blast design.
• Basis for conservative shot design when no monitoring is available.
• Site-specific relations should be developed via blast tests.

2. Scaling of Peak Particle Velocity (PPV) (Clause 6.1)

• Square-root scaling is traditional:

  [ \text{Scaled distance } n = \frac{R}{\sqrt{W}} ]

  where:

  • ( R ) = distance from blast (m)
  • ( W ) = charge weight per delay (kg)

• Alternative cube-root scaling (energy-based) can also be used.

• Empirical scaling exponent ( n ) often ranges between 0.4 to 0.6:

  [ n = \frac{R}{W^x}, \quad x = 0.4 \text{ to } 0.6 ]

3. Effects of Blast Geometry and Timing (Clause 15.2)

• Larger burden → higher intercept on velocity axis (higher PPV).
• Initiation timing affects frequency content and time history of vibrations.

4. Frequency Content (Clause 6.3)

• Dominant frequency decreases with distance.
• Lower frequencies dominate at larger distances, causing greater structural response.

Summary Table: Typical Scaling Relations

graph LR
A[Charge Weight per Delay (W)] --> B[Scaled Distance (n)]
B --> C[Peak Particle Velocity (PPV

Blast Vibration and Air Over-Pressure Propagation
(IS 14881: Clauses 2.2, 7.2, 7.3, 16.3)

Key Concepts

• Air over-pressure from blasts includes audible high-frequency and inaudible low-frequency waves that excite structures causing rattling.
• Over-pressure propagation scales with the cube-root of distance.
• Peak air over-pressure in decibels (dB) is defined as:
  [ \text{dB} = 20 \log_{10} \left(\frac{P}{P_0}\right) ]
  where (P_0 = 20 \times 10^{-6} \text{ Pa}) (reference pressure).

Table: Safe Air Over-Pressure Levels (Clause 16.3)

¹ C-weighted scale is less sensitive at low frequencies, thus more restrictive.

Important Notes

• Limits are based on wall strain equivalent to 19 mm/s peak particle velocity in ground motion.
• Increasing allowable over-pressure by 3 dB corresponds to 25 mm/s particle velocity.
• Broken glass typically occurs at 136-140 dB (linear transducer).
• Use of A-weighted scales is not recommended due to insensitivity to low frequencies critical for structural response.

Summary Diagram: Air Over-Pressure Wave Interaction

graph LR
A[Blast Explosion] --> B[Air Over-Pressure Wave]
B --> C[High Frequency (Audible Sound)]
B --> D[Low Frequency (Structural Excitation)]
D --> E[Wall Vibrations & Rattling Noise]
E --> F[Complaints & Possible Damage]

Use this data to monitor and control blast-induced air over-pressures to prevent structural damage and nuisance noise.

IS 14881: Measurement Techniques and Instruments - Key Points

1. Measurement Instruments & Calibration (Clauses 7.6 & 12.5)

• Instruments must record time history for frequency analysis; peak particle velocity alone is insufficient.
• Calibration is essential and periodic, using:
  • Manufacturer calibration curves (similar to transducer response spectra, see Fig. 6 in the code).
  • Special platforms controlling frequency & displacement.
  • Field calibrating circuits pulsing the geophone magnetic core.

2. Number and Type of Instruments (Clause 14.2)

• Type II instruments:
  • Continuously record peak particle velocity on one vertical axis (best for surface waves).
  • May or may not measure air overpressure.
  • Positioned beyond the nearest structure to cover a larger area.

3. Measurement & Analysis (Clause 8.3)

• Ground motion & air overpressure time histories → Calculate relative displacement → Calculate strain.
• Accuracy depends on:
  • Structure approximating a single-degree-of-freedom system.
  • Correct dynamic response characteristics.

Summary Table: Instrument Requirements

Calibration Concept Diagram

graph LR
A[Instrument] --> B[Calibration Curves]
B --> C[Platform Calibration]
B --> D[Field Calibration Circuit]

Note: Use light-sensitive paper or dot matrix printers for immediate frequency interpretation without extra equipment.

IS 14881: Measurement of Particle Velocity - Key Points

1. Preferred Parameter

• Particle velocity is preferred for describing ground motion due to its strong correlation with blast-induced cracking.
• It can be integrated to obtain displacement.
• Acceleration should be measured directly to avoid errors from differentiation.

2. Measurement Setup (Fig. 5)

• Use 3 orthogonal velocity transducers.
• Record data via tape, disk, or memory.
• Output via light beam oscilloscope or dot matrix printer.

3. Permissible Particle Velocity Limits (Clause 16.1.1)

4. Structural Considerations

• Relative displacement and strain measurements complement particle velocity data.
• Accuracy depends on the structure behaving like a single-degree-of-freedom system.

Formula for Displacement from Particle Velocity

[ d(t) = \int v(t) dt ]

where

• ( d(t) ) = displacement
• ( v(t) ) = particle velocity

flowchart LR
    A[3 Orthogonal Velocity Transducers] --> B[Recorder (Tape/Disk/Memory)]
    B --> C[Data Output (Oscilloscope/Printer)]
    C --> D[Analysis: Particle Velocity → Displacement]
    D --> E[Structural Response & Strain Estimation]

Summary: Measure particle velocity using 3 orthogonal sensors; apply limits based on frequency and structure type; integrate velocity for displacement; measure acceleration directly if needed.

Frequency Response of Transducers (IS 14881 Key Points)

• Definition (Clause 10.1):
  Frequency response is the range where the transducer output remains constant for constant mechanical input, typically within ±3 dB (±30% voltage variation).

  • Example: Linear within 3 dB between 2 Hz and 200 Hz means output voltage varies ≤30% in this range.
  • Use response spectra (e.g., Fig. 6) to identify frequency limits.

• Blast Vibration Specifics (Clause 10.2):

  • True blast phenomena span a wide frequency range:
    • Delayed gas pressure pulses: <1 Hz
    • Close-in accelerations: >100 Hz
  • No single transducer covers entire range; select based on key motion characteristics.

• Calibration (Clause 12.5):

  • Frequency analysis requires time-history records, not just peak values.
  • Calibration curves (similar to Fig. 6) must be periodically verified using controlled frequency-displacement platforms or magnetic core pulsing for geophones.

Typical Frequency Response Specification (Example)

Notes on Fig. 6 (Velocity Transducer Response)

• Shows output voltage vs. frequency for different damping levels.
• 70% critical damping offers a balanced flat response over a useful frequency range.

graph LR
A[Mechanical Motion Input] --> B[Transducer]
B --> C[Electrical Output Voltage]
C --> D[Frequency Response Curve]
D --> E{Within ±3 dB?}
E -->|Yes| F[Valid Frequency Range]
E -->|No| G[Outside Frequency Range]

Summary: Use transducers with known calibrated frequency response curves, ensure time-history data for frequency analysis, and select transducers based on the specific blast vibration frequency range of interest.

IS 14881: Key Points on Transducer Attachment

1. Mounting Depth & Surface (Clause 1.0)

• Soil surface: Transducer must be buried ≥ 15 cm below ground.
• Avoid mounting on spikes in soil to prevent free response errors.
• Rock/asphalt/concrete: Use double-sided tape, epoxy, or quick-setting cement.
• For accelerations > 1.0 g, cement or bolts only.
• Vertical surface mounts must be bolted.

2. Mounting Importance vs. Acceleration (Clause 11.1)

• For vertical accelerations < 0.2g, simple placement on horizontal surface is acceptable.
• For accelerations > 0.2g, more secure mounting is necessary to avoid rocking.

3. Frequency & Response Range (Clauses 10.1 & 10.3)

• Frequency response linear within ±3 dB between 2–200 Hz.
• Dominant excitation frequencies for structures: 5–100 Hz.
• Use special transducers if frequencies fall outside this range.

Summary Table: Transducer Attachment Guidelines

flowchart TD
    A[Measurement Surface] -->|Soil| B[Buried ≥ 15 cm]
    A -->|Rock/Asphalt/Concrete| C{Acceleration ≤ 1.0g?}
    C -->|Yes| D[Tape/Epoxy/Quick Cement]
    C -->|No| E[Cement or Bolts]
    F[Vertical Surface] --> G[Bolted Mounting]
    H[Acceleration < 0.2g] --> I[Simple Placement]
    H -->|≥ 0.

IS 14881 - Data Recording and Analysis: Key Points

1. Frequency Analysis (Clause 12.5)

• Requires time history records; peak particle velocity alone is insufficient.
• Permanent records via light-sensitive paper or dot matrix printers allow immediate frequency interpretation.
• Calibration curves provided by manufacturers resemble response spectra (see Fig. 6 in IS 14881).
• Recalibration needs special platforms controlling frequency and displacement or field calibrating circuits for geophones.

2. Recording Systems (Clauses 12.1 & 12.3)

• Use microprocessor/digital recording systems for:
  • Sampling rates: 500 to 1,000 samples/sec.
  • High accuracy; unaffected by tape speed variations.
  • Direct computer access for data analysis.
• Permanent records can be on:
  • Photographic film (UV-sensitive),
  • Floppy disks,
  • Battery memory chips,
  • Paper printouts.
• Consider printer reset times and environmental effects (e.g., cold weather).

3. Instrument Placement (Clause 11.2)

• Air over-pressure transducers:
  • Mounted ≥1 m above ground,
  • Pointed downward,
  • Covered with wind screens.

Calibration & Frequency Response Summary

flowchart TD
    A[Ground Vibrations] --> B[Transducers]
    B --> C[Signal Conditioning]
    C --> D[Digital Recorder]
    D --> E[Data Storage]
    E --> F[Frequency Analysis]
    F --> G[Calibration Check]

This ensures reliable frequency analysis and permanent record keeping as per IS 14881.

IS 14881: Calibration of Instruments — Key Points

Calibration Requirements (Clause 7.6 & 12.5)

• Periodic calibration of the entire vibration measurement system is mandatory.
• Calibration curves provided by manufacturers resemble response spectra for transducers (see Fig. 6 in IS 14881).
• Recalibration methods:
  • Controlled frequency and displacement platforms.
  • Field calibration using a circuit to pulse the magnetic core of geophones.
• Instruments recording only peak particle velocity cannot be used for frequency analysis; permanent records (e.g., light-sensitive paper or dot matrix printers) are needed.

Calibration Procedure Summary

Additional Notes

• Spare instruments (Clause 14.3) must be Type I if air over-pressure or frequency accuracy is critical.
• Round off calibration results per IS 2:1960 rules, matching significant figures of standard values.

flowchart TD
    A[Start Calibration] --> B[Use Calibration Platform]
    B --> C[Apply Known Frequency & Displacement]
    C --> D[Record Instrument Response]
    D --> E{Compare with Manufacturer's Curve}
    E -->|Match| F[Calibration OK]
    E -->|Mismatch| G[Recalibrate or Repair]
    F --> H[Document Calibration]
    G --> H

This ensures reliable, traceable vibration measurement consistent with IS 14881.

IS 14881: Blast Vibration Monitoring System Design & Deployment

Key Components (Clause 8.1)

• Transducers: Convert vibration motion/pressure to electric signals.
• Cables: Transmit signals to amplifiers.
• Amplifying System: Boosts signal strength.
• Recording Devices: Magnetic tape, paper, or digital recorders capture time variation.
• Display/Output: Light beam oscilloscope or dot matrix printer for visualization.

Important Specifications on Transducer Mounting (Clause 11.1)

• For vertical particle acceleration < 0.2g:
  • Transducers can be placed on horizontal surfaces without additional holding force.
• For vertical particle acceleration between 0.2g and higher:
  • Secure mounting is critical to avoid rocking or movement errors.

General Design Notes

• Use microprocessor-based systems for data acquisition, storage, and analysis.
• Ensure stable and rigid mounting of transducers for accurate vibration measurement.
• Battery-operated (12 V) portable systems are common for field use.

Typical Monitoring System Block Diagram

flowchart LR
    A[Transducer] --> B[Cable]
    B --> C[Amplifier]
    C --> D[Recorder (Magnetic Tape / Digital)]
    D --> E[Display (Oscilloscope / Printer)]

Additional Tips

• Use low-noise cables and shielded connectors.
• Calibrate transducers regularly.
• Position sensors close to blast source for accurate readings.

IS 14881: Factors Affecting Blast Vibration

Key Points from IS 14881 (Clauses 5.5, 6, 6.3)

• Dominant Frequency:

  • Surface mining blasts (large explosions, distant structures) → lower dominant frequencies.
  • Construction blasts (smaller explosions, close structures) → higher dominant frequencies.
  • Lower frequencies cause greater structural response and potential damage.

• Attenuation of Vibration:

  • Particle velocity decreases with increasing distance.
  • Attenuation follows a trend with square-root scaled distance:
    [ R / \sqrt{W} ] where:
    • ( R ) = distance from blast (m)
    • ( W ) = charge weight (kg)

• Scatter in Data:

  • Variability due to geology, blast design, and interference effects.
  • Confined shots (e.g., presplitting) produce higher particle velocities.

Typical Attenuation Relationship (from Fig. 3)

Practical Formula for Peak Particle Velocity (PPV):

[ \text{PPV} = k \times \left(\frac{W^{1/2}}{R}\right)^n ]

• ( k, n ) = site and blast-specific constants (from empirical data)
• ( W ) = charge weight (kg)
• ( R ) = distance from blast (m)

Summary Diagram (Mermaid.js):

graph LR
A[Blast Type] --> B[Dominant Frequency]
B --> C[Lower Frequency (Mining) → Higher Structural Response]
B --> D[Higher Frequency (Construction) → Lower Structural Response]
E[Distance from Blast] --> F[Particle Velocity ↓ with Distance]
F --> G[Attenuation follows R/√W scaling]
H[Geology & Blast Design] --> F

Note: For detailed monitoring and vibration limits, refer to IS guidelines on blast vibration monitoring methods.

IS 14881 Safety Criteria - Key Points

1. Air Over-Pressure Limits (Clause 16.3)

Limits are based on wall strain equivalent to 19 mm/s peak particle velocity ground motion.

¹ C-weighed has almost flat low-frequency response but requires more restrictive limits.

• Note: Using 25 mm/s velocity equivalence increases allowable over-pressure by 3 dB.
• A-weighted scales cannot be used due to insensitivity to low frequencies critical for structural response.

2. Attenuation Relations (Clause 15.2 & Annex A)

• Attenuation depends on blast geometry, timing, and site geology.
• Use Curve P (Fig. 3 in IS 14881) for conservative design when no monitoring.
• Site-specific attenuation relations should be established from blast tests.

Summary Formula for Safe Over-Pressure Level:

[ L_{max} = \text{Refer Table 1 based on instrument frequency response} ]

flowchart LR
    A[Blast Parameters] --> B[Attenuation Relation]
    B --> C{Site-Specific or Curve P}
    C --> D[Safe Change Weight per Delay]
    D --> E[Safe Air Over-Pressure Limits]
    E --> F[Wall Strain Equivalent to 19 mm/s Particle Velocity]

Use these criteria to ensure blast safety and minimize structural damage due to air-blast over-pressure.